pdfaoa / CH21

; Since we just saw a zero, write all ones to the output port.

; Now wait for all ones to arrive from the transmitting site.

;If we’ve timed out, check to see if the user has pressed ctrl-C to

;abort.

;Jump down here once we’ve seen both a zero and a one. Send the two

;in combinations until we get a 05h from the transmitting site or the

;user presses Ctrl-C.

; Okay, we’re synchronized. Return to the caller.

;GetFileInfoThe transmitting program sends us the file length and a

;zero terminated filename. Get that data here.

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; First four bytes contain the filesize:

; The next n bytes (up to a zero terminating byte) contain the filename:

;GetFileDataReceives the file data from the transmitting site

;and writes it to the output file.

;We’ve got more than 512 bytes left to go in this file, read 512 bytes

;at this point.

; Okay, write the data to the file:

; Process the last block, that contains 1..511 bytes, here.

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; Here’s the main program that gets the whole ball rolling.

; First, get the address of LPT1: from the BIOS variables area.

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21.5Summary

The PC’s parallel port, though originally designed for controlling parallel printers, is a general purpose eight bit output port with several handshaking lines you can use to control many other devices in addition to printers.

In theory, parallel communications should be many times faster than serial communications. In practice, however, real world constraints and economics prevent this from being the case. Nevertheless, you can still connect high performance devices to the PC’s parallel port.

The PC’s parallel ports come in two varieties: unidirectional and bidirectional. The bidirectional versions are available only on PS/2s, certain laptops, and a few other machines. Whereas the eight data lines are output only on the unidirectional ports, you can program them as inputs or outputs on the bidirectional port. While this bidirectional operation is of little value to a printer, it can improve the performance of other devices that connect to the parallel port, such as disk and tape drives, network adapters, SCSI adapters, and so on.

When the system communicates with some other device over the parallel port, it needs some way to tell that device that data is available on the data lines. Likewise, the devices needs some way to tell the system that it is not busy and it has accepted the data. This requires some additional signals on the parallel port known as handshaking lines. A typical PC parallel port provides three handshaking signals: the data available strobe, the data taken acknowledge signal, and the device busy line. These lines easily control the flow of data between the PC and some external device.

In addition to the handshaking lines, the PC’s parallel port provides several other auxiliary I/O lines as well. In total, there are 12 output lines and five input lines on the PC’s parallel port. There are three I/O ports in the PC’s address space associated with each I/O port. The first of these (at the port’s base address) is the data register. This is an eight bit output register on unidirectional ports, it is an input/output register on bidirectional ports. The second register, at the base address plus one, is the status register. The status register is an input port. Five of those bits correspond to the five input lines on the PC’s parallel port. The third register (at base address plus two) is the control register. Four of these bits correspond to the additional four output bits on the PC, one of the bits controls the IRQ line on the parallel port, and a sixth bit controls the data direction on the birdirectional ports.

For more information on the parallel port’s hardware configuration, see:

•“Basic Parallel Port Information” on page 1199

•“The Parallel Port Hardware” on page 1201

Although many vendors use the parallel port to control lots of different devices, a parallel printer is still the device most often connected to the parallel port. There are three ways application programs commonly send data to the printer: by calling DOS to print a character, by calling BIOS’ int 17h ISR to print a character, or by talking directly to the parallel port. You should avoid this last technique because of possible software incompatibilities with other devices that connect to the parallel port. For more information on printing data, including how to write your own int 17h ISR/printer driver, see:

•“Controlling a Printer Through the Parallel Port” on page 1202

•“Printing via DOS” on page 1203

•“Printing via BIOS” on page 1203

•“An INT 17h Interrupt Service Routine” on page 1203

One popular use of the parallel port is to transfer data between two computers; for example, transferring data between a desktop and a laptop machine. To demonstrate how to use the parallel port to control other devices besides printers, this chapter presents a program to transfer data between computers on the unidirectional parallel ports (it also works on bidirectional ports). For all the details, see

•“Inter-Computer Communications on the Parallel Port” on page 1209

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